The present invention pertains to a multi-element antenna which, in a receiving mode, is capable of distinguishing the direction from which signals are received without being mechanically steered and, in a transmitting mode, is capable of being electrically steered or radiating uniformly.
A three-element directional antenna is described in an article, "A Compact Transportable HF Radar System for Directional Coastal Wave Field Measurements," Barrick et al. in Ocean Wave Climate, Earle et al., ed., Plenum Publishing Corporation, 1979, pp. 153-163, and which is adapted for use as a receiving antenna in a wave height measuring system. This system requires a receiving antenna which is capable of distinguishing the direction from which signals reflected from ocean waves are received. The antenna itself is composed of two crossed-loop antennas insulatingly mounted above a monopole antenna which in turn is fed against a four-element radial ground plane. The two crossed-loop antennas and the monopole antenna are connected to separate coaxial cables, and the output signals from the three coaxial cables processed and analyzed to determine wave height information based upon the Doppler spectrum of signals returned from the sea surface.
Although this antenna is capable of performing the desired function to some extent, it suffers from serious drawbacks. Most serious among these is the fact that the loop antennas are 21 dB less efficient than the monopole. The outputs from the loop antennas and the monopole must be balanced so that the three signal levels will be equivalent in magnitude, and hence so that the quantization error is the same for all three antennas, in order to perform the desired signal processing. Balancing the loop antennas and the monopole antenna effectively requires that the output signal from the monopole antenna be reduced by 21 dB. Unfortunately, by attenuating the output of the monopole antenna, the system signal-to-noise ratio is accordingly reduced. This drastically reduces the effective range of the system.
Also, quite importantly, this antenna cannot be used for transmitting; due to the use of sequential selection of the loop and monopole antennas, only receiving is possible.
A similar antenna is described in U.S. Pat. No. 3,882,506 issued May 6, 1975 to Mori et al. This antenna includes a whip or monopole antenna which extends through and above two crossed-loop antennas. The vertical monopole antenna is electrically insulated from the crossed loops.
That antenna too suffers from a number of drawbacks. First, the effective height of the vertical antenna is only that portion which extends above the crossed loops. Thus, it may be necessary to make the overall vertical height of the antenna quite high, which may be unacceptable for some applications. Secondly, the antenna of Mori et al. is useful only in receiving applications. This is due to the fact that the vertical antenna must pass through the high impedance (and hence high voltage in the transmitting mode) portion of the loop antennas, specifically, the gap at the top of the loop antennas. Thus, high voltage flashover would likely occur if the antenna were used in a transmitting mode. Moreover, the transmitting efficiency of the loops is quite low.
U.S. Pat. No. 3,588,905 issued June 28, 1971 to Dunlavey describes a wide range tunable transmitting loop antenna composed of a single turn tuned primary loop fed by a small single turn untuned secondary. By properly adjusting the ratio between the diameters of the primary and secondary loops, an accurate impedance match to a low-impedance cable is achievable over a wide tuning range. Also, high efficiencies are attainable with relatively small sizes. For instance, a minimum efficiency of 30 percent is attainable for an overall antenna diameter of about five feet for a frequency range of 3 to 15 megahertz.
However, the antenna described by Dunlavey requires a mechanical rotator for steering. Moreover, this antenna cannot radiate omnidirectionally.